1. Field of the Invention
The present invention relates to an electronic display (electro optical device) formed by fabricating an EL (electro luminescence) element on a substrate. Particularly, the present invention relates to a display device using a semiconductor element (an element employing a semiconductor thin film), and furthermore to electronic equipment using the EL display device as a display portion.
2. Description of the Related Art
In recent years, remarkable progress has been made in a technique for forming thin film transistors (hereinafter referred to as TFTs) on a substrate, and developing the application of TFTs to an active matrix display device is proceeding. TFTs using a polycrystalline semiconductor film such as poly-silicon film, in particular, have a higher electric field effect mobility (also referred to as mobility) than that of conventional TFTs using an amorphous semiconductor film such as an amorphous silicon film, and hence a high speed operation may be made. Thus, control of pixels, which in the past has been controlled by a driver circuit external to a substrate, can now be made by driver circuits formed on the same substrate as the pixels.
Various merits such as reduction of manufacturing cost, miniaturization of a display device, and increase of yield and reduction of throughput can be obtained from such an active matrix display device using a polycrystalline semiconductor film by forming various circuits and elements on the same substrate.
A research on active matrix EL display devices having an EL element as a self-luminous element is being actively carried out. The EL display device is also referred to as an organic EL display (OLED) or an organic light emitting diode (OLED).
The EL element has a structure composed of a pair of electrodes (anode and cathode) and an EL layer, which is usually a laminate structure, sandwiched therebetween. The laminate structure (hole transporting layer, light-emitting layer, electron transporting layer) proposed by Tang, et al. from Eastman Kodak Company can be cited as a typical laminate structure of the EL layer. This laminate structure has an extremely high luminescence efficiency, and therefore at present, most of the EL display devices in which research and development are proceeding adopt this laminate structure of the EL layer.
In addition to the above laminate structure, a structure in which the layers are laminated on the anode in the order of a hole injection layer, a hole transporting layer, a light-emitting layer, and an electron transporting layer or in the order of a hole injection layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, and an electron injection layer may be formed. The light-emitting layer may be doped with a fluorescent pigment or the like.
The EL layer is a generic ten in the present specification indicating all the layers formed between the cathode and anode. Therefore, the above-mentioned hole injection layer, the hole transporting layer, the light-emitting layer, the electron transporting layer, the electron injection layer, etc. are all included in the EL layer.
If a predetermined voltage from the pair of electrodes is applied to the EL layer having the above structure, a re-coupling of carriers in the light-emitting layer occurs to thereby emit light. It is to be noted that throughout the present specification, the emission of light by the EL element is called a drive by the EL element. In addition, a luminescent element formed of the anode, the EL layer, and the cathode is called the EL element in the present specification.
It is to be noted that an EL element as used herein includes one utilizing light emission from singlet excited state (fluorescence) and one utilizing light emission from triplet excited state (phosphorescence).
A driving method of the analog system (analog drive) can be cited as a driving method of the EL display device. An explanation regarding the analog drive of the EL display device will be made with references to FIGS. 18 and 19.
FIG. 18 is a diagram showing the structure of a pixel portion 1800 in the EL display device having the analog drive. A gate signal line (G1 to Gy) for inputting a selected signal from a gate signal line driver circuit is connected to a gate electrode of a switching TFT 1801 of the respective pixels. As to a source region and a drain region of the switching TFT 1801 of the respective pixels, one is connected to a source signal line (also called data signal line) (S1 to Sx) for inputting an analog video signal whereas the other is connected to a gate electrode of a driver TFT 1804 and a capacitor 1808 of each of the pixels, respectively.
A source region and a drain region of the driver TFT 1804 of each of the pixels is connected to power supply lines (V1 to Vx), and a drain region thereof is connected to an EL element 1806, respectively. An electric potential of the power supply lines (V1 to Vx) is called a power supply potential. Each of the power supply lines (V1 to Vx) is connected to the capacitor 1808 of the respective pixels.
The EL element 1806 is composed of an anode, a cathode, and an EL layer sandwiched therebetween. When the anode of the EL element 1806 is connected to either the source region or the drain region of the EL driver TFT 1804, the anode and the cathode of the EL element 1806 become a pixel electrode and an opposing electrode, respectively. Alternatively, if the cathode of the EL element 1806 is connected to either the source region or the drain region of the EL driver TFT 1804, then the anode of the EL element 1806 becomes the opposing electrode whereas the cathode thereof becomes the pixel electrode.
It is to be noted that the electric potential of an opposing electrode is herein referred to as opposing potential. It is to be noted that a power supply for giving opposing potential to an opposing electrode is herein referred to as an opposing power supply. The difference between the electric potential of a pixel electrode and the electric potential of an opposing electrode is the EL driving voltage, which is applied to the EL layer.
FIG. 19 is a timing chart illustrating the EL display shown in FIG. 18 when it is being driven by the analog system. A period from the selection of one gate signal line to the selection of a next different gate signal line is called a 1 line period (L). In addition, a period from the display of one image to the display of the next image corresponds to a 1 frame period (F). In the case of the EL display device of FIG. 18, there are y number of the gate signal lines and thus a y number of line periods (L1 to Ly) are provided in 1 frame period.
Because the number of line periods in 1 frame period increases as resolution becomes higher, driver circuits must be driven at a high frequency.
First of all, the power supply lines (V1 to Vx) are held at a constant power supply potential, and the opposing electric potential that is the electric potential of the opposing electrode is also held at a constant electric potential. There is a difference in electric potential between the opposing electric potential and the power supply potential to a degree that the EL element can emit light.
A selected signal from the gate signal line driver circuit is inputted to the gate signal line G1 in the first line period (L1). An analog video signal is then sequentially inputted to source signal lines (S1 to Sx). All the switching TFTs connected to the gate signal line G1 are turned on to thereby input the analog video signal that is inputted to the source signal lines to the gate electrode of the driver TFT through the switching TFT.
The amount of electric current through the channel forming region of a TFT for driving is controlled by its gate voltage.
Here, description is made with regard to, by way of example, a case where the source regions of the TFTs for driving are connected to the power supply lines and the drain regions of the TFTs for driving are connected to the EL elements.
Since the source regions of the TFTs for driving are connected to the power supply lines, the same electric potential is inputted to the respective pixels of the pixel portion. At this point, when an analog signal is inputted to a source signal line, the difference between the electric potential of the signal voltage and the electric potential of the source region of the TFT for driving becomes the gate voltage. The electric current through an EL element depends on the gate voltage of the TFT for driving. Here, the brightness of the emitted light from an EL element is proportional to the electric current between the electrodes of the EL element. In this way, the EL elements emit light under the control of the voltage of analog video signals.
The operation described in the above is repeated. When input of analog video signals to the source signal lines (S1-Sx) is completed, the first line period (L1) ends. It is to be noted that the period until the input of analog video signals to the source signal lines (S1-Sx) is completed combined with a horizontal retrace line period may be a one line period. Then, in the second line period (L2) that follows, a selection signal is inputted to the gate signal line G2. Similarly to the case of the first line period (L1), analog video signals are sequentially inputted to the source signal lines (S1-Sx).
When selection signals are inputted to all the gate signal lines (G1-Gy), all the line periods (L1-Ly) end. When all the line periods (L1-Ly) end, one frame period ends. In one frame period, all the pixels carries out display to form one image. It is to be noted that all the line periods (L1-Ly) combined with a vertical retrace line period may be a one frame period.
As described in the above, the amount of light emitted from an EL element is controlled by an analog video signal, and, by controlling the amount of light emission, gradation display is carried out. This is a so-called analog driving method, where gradation display is carried out by changing the voltage of analog video signals inputted to the source signal lines.
FIG. 20 is a graph illustrating characteristics of a TFT for driving. 401 is referred to as Id-Vg characteristics (or an Id-Vg curve), wherein Id is drain current and Vg is gate voltage. Using this graph, the amount of electric current with regard to arbitrary gate voltage can be known.
In driving an EL element, a region shown by a dotted line 402 of the above Id-Vg characteristics is normally used. The region surrounded by the dotted line 402 is referred to as a saturated region where the drain current Id greatly changes as the gate voltage Vg changes.
In the analog driving method, using the saturated region, the drain current of a TFT for driving is changed by changing its gate voltage.
When a TFT for switching is turned on, an analog video signal inputted from a source signal line to a pixel is applied to a gate electrode of a TFT for driving. In this way, the gate voltage of the TFT for driving is changed. Here, according to the Id-Vg characteristics illustrated in FIG. 20, the drain current with regard to a certain gate voltage is uniquely decided. In this way, predetermined drain current corresponding to the voltage of the analog video signal inputted to the gate electrode of the TFT for driving passes through the EL element, and the EL element emits light the amount of which corresponds to the amount of the electric current.
In this way, the amount of light emitted from the EL element is controlled by an analog video signal, and, by controlling the amount of light emission, gradation display is carried out.
Here, even when the same signal is inputted from the source signal line, the gate voltage of the TFT for driving of each pixel changes if the electric potential of the source region of the TFT for driving changes. Here, the electric potential of the source region of the TFT for driving is given from the power supply line. However, due to potential drop caused by wiring resistance, the electric potential of the power supply line changes depending on its position in the pixel portion.
In addition to the influence of the potential drop caused by wiring resistance of the power supply line in the pixel portion, there is also a problem of potential drop of the connection wiring portion (hereinafter referred to as a power supply line connection wiring portion) from an input portion of the power supply from the external (hereinafter referred to as an external input terminal) to the power supply line of the pixel portion.
More specifically, depending on the length of the wiring from the position of the external input terminal to the position of the power supply line of the pixel portion, the electric potential of the power supply line varies.
Here, this may not present a great problem in such a case where the wiring resistance of the power supply line is small, the display device is relatively small, or the amount of electric current passing through the power supply line is relatively small. Otherwise however, especially when the display device is relatively large, the change in the electric potential of the power supply line due to the wiring resistance becomes large.
In particular, as the display device becomes larger, the variation in the distance from the external input terminal to the power supply line of the pixel portion becomes larger, and the variation in the length of the wiring of the power supply line drawn-around portion becomes larger accordingly. Therefore, the change becomes larger in the electric potential of the power supply line due to the potential drop of the power supply line connection portion.
The variation in the electric potential of the power supply lines due to these factors affects the brightness of the emitted light from the EL elements of the pixels by changing the brightness of the display, and thus, is a cause of uneven display.
A specific example of such variation in the electric potential of the power supply lines is described in the following.
As illustrated in FIG. 23, when a white or black box is displayed on a display, a phenomenon referred to as cross talk arises. This is a phenomenon that difference in the brightness arises over or under the box compared with portions beside the box.
FIGS. 40 and 41 are a partial circuit diagram and a top surface view, respectively, of a pixel portion of a conventional display device where the phenomenon arises.
In FIG. 41, like reference numerals designate like parts in FIG. 40, and the description thereof is omitted.
Each pixel is formed of a TFT 4402 for switching, a TFT 4406 for driving, a storage capacitor 4419, and an EL element 4414.
It is to be noted that, although the TFT 4402 for switching is of a double gate structure in FIGS. 40 and 41, it may be of the other structures.
Cross talk arises due to the difference in electric current through the TFT 4406 for driving between each pixel over and under the box and beside the box. The difference arises because the power supply lines V1 and V2 are disposed in parallel with the source signal lines S1 and S2.
For example, as shown in FIG. 23, when a white box is displayed in a part of the display in the power supply line corresponding to the pixel displaying the box, since current flows through the EL element between the source and the drain of the TFT for driving of the pixel displaying the box, the potential drop due to the wiring resistance of the power supply line is greater than that of the power supply line which supplies power only to pixels which do not display the box. Therefore, portions darker than other pixels which do not display the box are generated over and under the box.
Further, in a conventional active matrix EL display device, as shown in FIG. 24, the power supply line is drawn out from one direction of the display device, and power supply, signals, and the like are inputted from an input portion.
Here, even if the size of the display of the display device is small, no particular problem arises. However, as the size of the display of the display device becomes larger, the current consumption increases in proportion to the area of the display.
For example, the current consumption of a display device having a 20-inch display is 25 times as much as that of a display device having a 4-inch display.
Therefore, the potential drop described in the above is a big problem for a display device having a large-sized display.
Further, while the potential drop with regard to a power supply line near the input portion (a in FIG. 24) is not so great, with regard to a power supply line far from the input portion (b in FIG. 24), since the length of the wiring is large, the potential drop caused due to the wiring resistance is large. Therefore, voltage applied to EL elements of pixels having TFTs for driving which are connected to the power supply line (b in FIG. 24) is lowered to deteriorate the quality of the image.
For example, in a 20-inch display device, when the length of the wiring is 700 mm, the width of the wiring is 10 mm, and the sheet resistance is 0.1 ohm, if about 1 A of current passes, the potential drop is as much as 10 V, with which normal display is impossible.